Hydraulic system having at least one hydraulic valve for controlling a transmission

ABSTRACT

A hydraulic system having at least one hydraulic valve for controlling a transmission, in particular a continuously variable transmission. The hydraulic valve is controlled by a pilot circuit including at least one actuator having a variable flow resistance and one actuator having a constant flow resistance. The actuator having a variable flow resistance is connected upstream of the actuator having a constant flow resistance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns a hydraulic system having at least onehydraulic valve for controlling a transmission, in particular acontinuously variable transmission, wherein the hydraulic valve can becontrolled by a pilot circuit having at least one actuator having avariable flow resistance and one actuator having a constant flowresistance.

2. Description of the Related Art

A hydraulic system of that sort is adequately known from motor vehicletechnology. Preferably, the hydraulic system can be used to control thecontrol components of a continuously variable transmission, which isalso called a CVT transmission.

For example, with the known hydraulic system an actuator having aconstant flow resistance and an actuator having a variable flowresistance are used in the pilot circuit. The pilot circuit ispressurized by a supply source with a predetermined supply pressure. Thesupply pressure acts first on the actuator having a constant flowresistance, with the actuator having a variable flow resistance beingconnected in series with the actuator having a constant flow resistancein the flow direction. The two flow resistances of the actuators therebyform a pressure divider. A control pressure conduit is connected to theconnecting conduit between the actuator having a constant flowresistance and the actuator having a variable flow resistance, whichpressurizes the hydraulic valve with a desired control pressure.

When controlling the CVT transmission, actuators with risingcharacteristic curves are usually used. For a variety of reasons, suchas safety aspects, for example, it can be simpler and can create fewercomplications if an actuator having a falling characteristic curve isinstalled in the hydraulic system. That results in the disadvantage thatit is not possible to employ the usual components, namely actuatorshaving rising characteristic curves.

An object of the present invention is to provide a hydraulic system ofthe type described above, in which a falling characteristic curve isimplemented and without an actuator having a falling characteristiccurve being utilized.

SUMMARY OF THE INVENTION

In accordance with the present invention, that object is achieved by ahydraulic system having at least one hydraulic valve for controlling atransmission, in particular a continuously variable transmission,wherein the hydraulic valve is controlled by a pilot circuit includingat least one actuator having a variable flow resistance and one actuatorhaving a constant flow resistance, and where, in accordance with theinvention, the actuator having a variable flow resistance is connectedupstream of the actuator having a constant flow resistance.

Accordingly, through the type of interconnection in accordance with theinvention, a falling characteristic curve is achieved in the hydraulicvalve to control a transmission, but an actuator having a risingcharacteristic curve is used. Thus a cost reduction is achieved throughthe use of like components, since as a rule actuators with risingcharacteristic curves are employed in that application.

In the case of the present invention, an advantageous embodimentprovides that a proportional valve with a rising characteristic curve beused as an actuator having a variable flow resistance. It is alsoconceivable, however, that other valves or similar components can beutilized as actuators, which achieve a similar or identical function asa proportional valve with a rising characteristic curve.

Preferably, in accordance with a further refinement of the presentinvention, a fixed orifice plate, or the like, is utilized as anactuator having a constant flow resistance. Here, also, the possibilityof using other components with a similar or identical function exists.The diameter of the orifice plate used can be, for example, 0.8 mm.Other dimensions are also conceivable.

In accordance with a refinement of the invention, the actuator having aconstant flow resistance can be connected to a leakage tank. Thus, whencontrolling the desired control pressure, any leakage flow that resultscan be conducted away to the leakage tank.

With the application of the hydraulic system in accordance with theinvention provided herein for controlling a CVT transmission, the pilotcircuit can preferably be pressurized with a constant pressure of about5 bar. However, other supply pressures are also possible.

The chosen supply pressure results in a possible control pressurebetween 0 and 5 bar.

When using a proportional valve with a rising characteristic curve as anactuator having a variable flow resistance, and a fixed orifice plate asan actuator having a constant flow resistance, it is possible toincrease the hydraulic flow resistance to a maximum when flow isoccurring in the proportional valve that is in the form of anelectromagnetic valve, the result of which is that the controllingcontrol pressure is low or takes the value 0 when the proportional valveis completely closed.

On the other hand, when there is no flow in the proportional valve, acontrol pressure which corresponds to the supply pressure can be presentat the hydraulic valve. In that way, in spite of the use of aproportional valve with a rising characteristic curve, a fallingcharacteristic curve of the hydraulic valve is possible in order toactuate transmission components accordingly.

BRIEF DESCRIPTION OF THE DRAWING

The structure, operation, and advantages of the present invention willbecome further apparent upon consideration of the following description,taken in conjunction with the accompanying drawing in which:

FIG. 1 shows a schematic view of the circuit of the actuators in thepilot circuit for controlling a hydraulic valve in accordance with theinventive hydraulic system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The proposed hydraulic system includes a pilot circuit 1. The pilotcircuit 1 has two actuators, one actuator having a variable flowresistance and designed as a proportional valve 2, which is operatedelectromagnetically. In addition, pilot circuit 1 includes as anactuator having a constant flow resistance a fixed orifice plate 3,which is connected to a leakage tank 4.

Proportional valve 2 is pressurized by a supply source 5 with a constantsupply pressure of e.g. 5 bar. A control pressure line 6 branches offfrom the connecting line between proportional valve 2 and fixed orificeplate 3. The control pressure line pressurizes a hydraulic valve 7 withthe particular control pressure desired. The control pressure can bebetween 0 and 5 bar in that exemplary embodiment. Hydraulic valve 7serves to control a transmission (not shown), in particular a CVTtransmission.

The fixed orifice plate has a diameter of about 0.8 mm in that exemplaryembodiment. The hydraulic valve to be controlled is connected betweenthe two actuators, and ideally has no leakage.

When there is a flow through proportional valve 2, the hydraulic flowresistance becomes very large, since the electromagnetically operatedproportional valve 2 is moved toward the closed direction by flow.Because of the increasing hydraulic flow resistance, the controlpressure is reduced until ultimately there is no longer any pressure atthe hydraulic valve and the control pressure corresponds to zero bar.

When there is no flow through proportional valve 2, the hydraulic flowresistance is correspondingly small, since proportional valve 2 isopened or is already open. As a result, the control pressure rises untila pressure of 5 bar is reached, if the proportional valve is fully open.

Although particular embodiments of the present invention have beenillustrated and described, it will be apparent to those skilled in theart that various changes and modifications can be made without departingfrom the spirit of the present invention. It is therefore intended toencompass within the appended claims all such changes and modificationsthat fall within the scope of the present invention.

1. A hydraulic system having at least one hydraulic valve forcontrolling a transmission, such as a continuously variabletransmission, said hydraulic system comprising: a hydraulic valve thatis actuated through a pilot circuit including at least one actuatorhaving a variable flow resistance and one actuator having a constantflow resistance, wherein the actuator having a variable flow resistanceis connected upstream of the actuator having a constant flow resistance.2. A hydraulic system in accordance with claim 1, wherein the actuatorhaving a variable flow resistance is a proportional valve having arising characteristic curve.
 3. A hydraulic system in accordance withclaim 1, wherein the actuator having a constant flow resistance is afixed orifice plate.
 4. A hydraulic system in accordance with claim 3,wherein the fixed orifice plate has a diameter of about 0.8 mm.
 5. Ahydraulic system in accordance with claim 1, wherein the actuator havinga constant flow resistance is connected to a leakage tank.
 6. Ahydraulic system in accordance with claim 1, wherein the pilot circuitis pressurized at a constant pressure of about 5 bar.